The invention relates to a method of reinforcing a paved surface, and also to a reinforcement for use in the method.
The invention is applicable to paved surfaces in general, but the invention is particularly useful when reinforcing asphalt. "Asphalt" is used herein in the general sense to refer to an aggregate having a hydrocarbon-type binder, which may be for instance natural asphalt or bitumen; the asphalt is normally laid hot and sets on cooling, though cold-mix asphalts can be used. The asphalt can be used on for instance a road or runway, normal practice being to lay an asphalt base course and to place an asphalt wearing course over the base or binder course. The ground is the surface to be covered with a paving layer, and can be any suitable surface, and can eventually be within a multi-layer construction, e.g. within the thickness of a full depth asphalt pavement or road. The ground can be e.g. the surface of a lower layer of asphalt or concrete to be covered with an upper layer of asphalt, or the base or binder course, or the lower part of an old wearing course when refurbishing, or a concrete or lean-mix concrete base or sub-base. Thus the invention can be applied when making a new construction or when refurbishing, for instance, after part or whole removal of the old wearing course, or even directly onto an old wearing course.
In recent years, mesh structures (which are also termed grids) have been incorporated in order to improve the paved surface. The mesh structure can control rutting and reflective cracking (cracking which is initiated in or below the base course). The mesh structure itself is laid onto the ground and is tensioned, the tension being maintained throughout the laying process. Modern practice is to spray the mesh structure and the ground with a bituminous emulsion or solvent-based bitumen known as "cut-back", and a layer of small stone chippings is then spread over the surface, becoming adhered to the bitumen. A conventional paving operation can then be carried out to apply a layer of hot asphalt onto the layer of chippings without the tracks or wheels of the asphalt laying machine (called a paviour) coming into contact with the grid. The layer of hot asphalt becomes integrated with the layer of chippings and bitumen, the whole composite being bonded to the road surface through the bitumen. Vertical loads on the new road surface are resolved into horizontal forces, which are then restrained by the grid structure into which the chippings and aggregate from the asphalt layer interlock.
Whilst this procedure is effective, it is complex and relatively expensive because of the number of individual operations necessary. The coating of the ground and mesh structure with bitumen and then spreading a chip coat, is expensive and time consuming.
The mesh structure itself is preferably formed of plastics material, high-strength molecularly oriented polymer structures being used, for instance, a biaxially-oriented mesh in accordance with U.S. Pat. No. 4,374,798. However, other materials such as glass fibre grids, woven or knitted polyester meshes or metal grids may be used.
A commercially-acceptable method is desirable for positioning the mesh structure in the asphalt. In one experimental procedure, the mesh structure was laid on top of the base or binder course and the wearing course placed on top of the mesh structure using a conventional machine; the mesh structure was a biaxially-orientated mesh structure in accordance with U.S. Pat. No. 4,374,798. It was found that if the mesh structure was merely laid on the surface of the base or binder course, the tractive forces and weight of the asphalt laying machine caused the mesh structure to stretch and buckle as the wearing course was laid, resulting in the mesh structure being imprecisely positioned and causing subsequent rucking of the mesh structure and splitting of the asphalt. The mesh structure moves forward in front of the wheels or track of the machine, forming a type of bow wave. This makes it impossible to determine the position of the mesh structure within the wearing course.